1. Field of the Invention
This invention relates to 5-keto-L-ascorbic acid-5,5-dimethyl ketal and to a process for its preparation by the reaction of methyl 2,5-diketogluconate-5,5-dimethyl ketal with an inorganic base in an alcohol or aqueous solvent system followed by neutralization of the 5-keto-L-ascorbic acid-5,5-dimethyl ketal inorganic salt thus produced. The 5,5-dimethyl ketal of 5-keto-L-ascorbic acid is an effective antioxidant, an antiscorbutic agent, and an intermediate for production of 5-keto-L-ascorbic acid, also an antiscorbutic agent, and L-ascorbic acid.
2. Description of the Prior Art
Finkle et al., Biochim. Biophys. Acta, 38, 332-339 (1960), proposed 5-keto-L-ascorbic acid as an intermediate in the biosynthesis of ascorbic acid in plants. However, no enabling disclosure as to its preparation is given, and no evidence has been found to data to support this proposal.
3. Summary of the Invention
This invention relates to 5-keto-L-ascorbic acid-5,5-dimethyl ketal and to a method for its preparation which comprises reacting methyl 2,5-diketogluconate-5,5-dimethyl ketal with an inorganic base in a solvent selected from the group consisting of water, alcohols having from one to four carbon atoms and mixtures of said solvents at a temperature of from about 20.degree. C. to the reflux temperature of the solvent. The inorganic salt of 5-keto-L-ascorbic acid-5,5-dimethyl ketal thus produced is then neutralized in a liquid medium to produce 5-keto-L-ascorbic acid-5,5-dimethyl ketal which, when treated with a strong acid, produces 5-keto-L-ascorbic acid, a compound embraced within this invention in its keto, hydrated and enol forms. Reduction of 5-keto-L-ascorbic acid with sodium cyanoborohydride or zinc cyanoborohydride in water at pH values of 2.5 to 2 affords L-ascorbic acid.